Direct drive schemes have been popular recently to power solid state lighting, such as Light Emitting Diode (LED) light bulbs, to avoid the cost or complexity of switching regulators which bring with them unwanted EMI filter and bill of material expenses. Various direct drive schemes have been suggested, however, they generally include utilizing several subsets of one or more series connected LEDs which are shorted or bypassed by switches to increase or decrease the total forward voltage in proportion to the AC input. Normally the switches used are MOSFETs which are turned on or off depending upon the AC input voltage at the time.
To improve power factor and reduce total harmonic distortion (THD), these types of switching schemes are used in conjunction with valley fill power factor correction schemes (VFPFC). These schemes break up the input capacitor into two or more capacitors to alter the effective input and discharge capacitance so as to distribute the input conduction angle over a wider range and spread the capacitance discharge over a wider conduction angle.
Unfortunately, these schemes underutilize expensive LEDs because many of them are only on for a portion of the conduction cycle rather than continuously as with more expensive switching regulator based schemes. Additionally, these schemes are not useful for voltage mode applications such as cell phone or portable equipment chargers which require a constant average output voltage.
Standard wall dimmers create phase control type dimming that is difficult to correlate with direct drive schemes since direct drive requires a continuous half wave rectified signal to make good use of the LEDs. Valley fill Power Factor Correction (PFC) schemes are poorly compatible with these types of schemes since they are generally passive or minimally active and not responsive to phase control type signals. Most direct drive schemes working with phase control dimmers will turn on only a small fraction of the available LEDs leaving much of a luminaire unilluminated. The eye is generally dissatisfied with the resulting bright spot in the luminaire rather than a continuous dimming across all of the LEDs in the physical LED array.
The basic limitation on improving the utilization of LEDs, or allowing more complex offline capacitor schemes is the cost of the switches. MOSFETs are expensive especially at high voltage and therefore much emphasis is placed on reducing the voltage on each MOSFET and in addition reducing the number of switches. This results in a number of problems including flicker, poor total harmonic distortion (THD), EMI, poor LED utilization & reduced power factor. More switches would reduce flicker, allow better power factor, improve total harmonic distortion (THD) & LED utilization, and reduce use of capacitors. Therefore, it would be desirable to have a mechanism that maximizes the conduction time of all LEDs. It would be especially desirable if it were also compatible with phase control dimming.
Alternatively, a mechanism compatible with a fixed voltage output by connecting capacitors groups in conformance with the rectified AC input voltage which switches said capacitors at frequencies much higher than the input sine wave frequency, from a series-parallel arrangement compatible with the instantaneous input voltage, to a series-parallel configuration compatible with an output or load voltage, would be desirable. Such an offline charge pump configuration compatible with portable equipment such as power supply or charger circuits also known as “wall warts,” common for cell phones, tablets or laptop computers, could greatly reduce the cost, size, component count, weight and overcome EMI disadvantages of existing circuits.
It would be desirable to have a switching matrix with superior properties to MOSFETs, bipolar transistors or even insulated gate bipolar transistors (IGBTs) such that more complex switching matrix for LEDs or capacitors might be utilized at reasonable cost. The best devices in terms of current density (see FIG. 9) are thyristors. These thyristors, however, must be capable of turning on and off without relying on the current to fall to zero as in normal thyristor circuits. This is accomplished utilizing Gate Turn Off (GTO) thyristors and/or Mos Controlled Thyristors (MCT) which can be turned off in response to a control signal.
Commercially available GTO and thyristors are only available in individual vertical form and generally only at high current levels. Additionally, no arrays or groups of turn off thyristors on the same silicon substrate which are isolated relative to one another have been demonstrated. The use of turn off thyristors for offline voltage mode applications requires such isolated devices as creating a substrate or package capable of dealing with multiple potentials is too expensive for offline LED drivers or power supplies. It would therefore be desirable to have a switch matrix based on turn off thyristors such as GTOs/MCTs/turn off TRIACs that can be made on a single piece of silicon as a switching array at low cost.
Finally, it would be desirable to have a scheme for level shifting the control signals that drive the control nodes of said turn off thyristors individually or in combinations of two in reverse parallel configuration (turn off TRIACs) such that the voltage and current limits on said control nodes are maintained (i.e. Safe Operating Area (SOA)) while simultaneously eliminating the need for high valued resistor based schemes with long time constants which reduce the number of times per cycle that switching can occur.
Therefore, a need exists to provide a device and method to overcome the above problems.